Electrical components come in various conventional forms, such as inductors, capacitors and resistors. A lumped electrical element is one whose physical size is substantially less than the wave length of the electromagnetic field passing through the element. A distributed element is one whose size is larger than that for a lumped element. As an example, a lumped element in the form of an inductor would have a physical size which is a relatively small fraction of the wave length used with the circuit, typically less than ⅛ of the wavelength.
Inductors, capacitors and resistors have been grouped together into useful circuits. Useful circuits including those elements include resonant circuits and filters. One particular application has been the formation of filters useful in the microwave range, such as above 500 MHZ.
Considering the case of conventional microwave filters, there have been basically three types. First, lumped element filters have used separately fabricated air wound inductors and parallel plate capacitors, wired together into a filter circuit. These conventional components are relatively small compared to the wave length, and accordingly, make for a fairly compact filters. However, the use of separate elements has proved to be difficult in manufacture, and resulting in large circuit to circuit differences. The second conventional filter structure utilizes mechanical distributed element components. Coupled bars or rods are used to form transmission line networks which are arranged as a filter circuit. Ordinarily, the length of the bars or rods is ¼ or ½ of the wave length at the center frequency of the filter. Accordingly, the bars or rods can become quite sizeable, often being several inches long, resulting in filters over a foot in length. Third, printed distributed element filters have been used. Generally they comprise a single layer of metal traces printed on an insulating substrate, with a ground plane on the back of the substrate. The traces are arranged as transmission line networks to make a filter. Again, the size of these filters can become quite large. The structures also suffer from various responses at multiples of the center frequency.
Various thin-filmed lumped element structures have been proposed. Swanson U.S. Pat. No. 4,881,050, issued Nov. 14, 1989, discloses a thin-film microwave filter utilizing lumped elements. In particular, a capacitor π network utilizing spiral inductors and capacitors is disclosed. Generally, a multi-layer structure is utilized, a dielectric substrate having a ground plane on one side of the substrate and multiple thin-filmed metal layers and insulators on the other side. Filters are formed by configuring the metal and insulation layers to form capacitive π-networks and spiral inductors. Swanson U.S. Pat. No. 5,175,518 entitled “Wide Percentage Band With Microwave Filter Network and Method of Manufacturing Same” discloses a lumped element thin-film based structure. Specifically, an alumina substrate has a ground plane on one side and multiple layer plate-like structures on the other side. A silicon nitride dielectric layer is deposited over the first plate on the substrate, and a second and third capacitor plates are deposited on the dielectric over the first plate.
Historically, such lumped element circuits were fabricated using normal, that is, non-superconducting materials. These materials have an inherent loss, and as a result, the circuits have various degree of lossiness. For resonant circuits, the loss is particularly critical. The quality factor Q of a device is a measure of its power dissipation or lossiness. Resonant circuits fabricated from normal metals have Q's at best on the order of a few hundred.
With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from these materials. The microwave properties of the high temperature superconductors has improved substantially since their discovery. Epitaxial superconductive thin films are now routinely formed and commercially available. See, e.g., R. B. Hammond, et al., “Epitaxial Tl2Ca1Ba2Cu2O8 Thin Films With Low 9.6 GHz Surface Resistance at High Power and Above 77 K”, Appl. Phy. Lett., Vol. 57, pp. 825–827, 1990. Various filter structures and resonators have been formed. Other discrete circuits for filters in the microwave region have been described. See, e.g., S. H. Talisa, et al., “Low- and High-Temperature Superconducting Microwave Filters,” IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 9, September 1991, pp. 1448–1554.
The need for compact, reliable narrow band filters has never been stronger. Applications in the telecommunication fields are of particular importance. As more users desire to use the microwave band, the use of narrow band filters will increase the number of users in the spectrum. The area from 800 to 2,000 MHZ is of particular interest. In the United States, the 800 to 900 MHz range is used for analog cellular communications. The personal communications services are planned for the 1,800 to 2,000 MHZ range.
Many passive microwave devices, for example, resonators, filters, antennas, delay lines and inductors, have been fabricated in planar form utilizing high temperature superconducting thin films. As described, such structures are often smaller than conventional technologies in terms of physical size. However, these devices are also limited in their size given the constraints of fabricating high quality, epitaxial films. As a result, devices fabricated in HTS films are often of a quasi-lumped element nature, that is, where the nominal size the device is smaller than the wavelength of operation. This often results in folding of devices, which leads to significant coupling between lines.
Despite the clear desirability of improved electrical circuits, including the known desirability of converting circuitry to include superconducting elements, efforts to date have been less than satisfactory in all regards. It has proved to be especially difficult in substituting high temperature superconducting materials to form circuits without severely degrading the intrinsic Q of the superconducting film. These problems include circuit structure, radiative loss and tuning and have remained in spite of the clear desirability of an improved circuit.